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Distribution: Fedora (workstations), CentOS (servers), Arch, Mint, Ubuntu, and a few more.
This is the simplest picture. Boot loader is the program wich tells your computer where and how to find your operating system.
LILO is relatively old (still is the default in Mandrake, Vector) GRUB is the newer and more powerful (default in Debian, Fedora, Red Hat) You may have a look at the Boot Loader Showdown Matirs link is also good.
By the way, LILO stands for LInux LOader and GRUB (not Grup) stands for GRand Unified Bootloader.
A boot loader is a small binary program that is designed to pull an operating system into the PC memorary. It is necessary because on power up the current BIOS design is to read the first boot sector (512 bytes and known as the MBR) of the first bootable hard disk.
Only the first 446 bytes belongs to the boot loader and the remaining 64 bytes are for the partition table. The last 2 bytes are spare.
Only DOS's boot loader is that smal. Nearly all later boot loaders are bigger in size and so a two-part process is needed to do a booting for a Linux or a Windows system.
Many PC user may not realise that the first sector of every partition is reserved for the boot loader and is immuned to the filing system. Thus one can format the partition but cannot remove its boot loader.
Boot loader can only be overwritten by special programs
In Windows they are fdisk /mbr, fixmbr, sys, format c:/s etc
In Linux to alter the boot loader the command for Lilo is
lilo -b /dev/hdax # where x=partition No.
in Grub the equivalent is
Linux also uses a less known boot loader called Loadin which is a DOS program. It loses its popularity because nowaday we have big hard drives which DOS has a problem to cope with them.
Both Lilo and Grub carry out their booting according to the command sstored in a text file; /etc/lilo.conf for the former and /boot/grub/menu.lst for the latter. Both boot loaders can co-exist with each other and boot each other in different Linux. Each needs 2 to 3 lines added to the text file for booting an additional entry.
Linux's boot loader are powerful because they can "chain-load" a system foreign to them. That is why Linux boot loaders can boot DOS, Windows, Win2k/XP, *BSD and Soloris. As both Lilo and Grub can boot systems before their own creation it can imply they are also capable of booting future systems haven't been invented on the PC! So they are definitely worth to know something about.
The following are the differences I am aware of the two
Maximum bootable entries = 15
Need a re-validation every time its /etc/lilo.conf is changed (just issuing "lilo" at root terminal will do)
Lilo refuse to update its /etc/lilo.conf if it finds any unbootable entry on the list.
Lilo does not provide a command prompt to work alone (unattached to an operating system)
The bootloader is sensitive to damage from partitions it has been asked to boot because its mandatory checking on their bootability everying the command "lilo" is run. This happen every time if you see L and a screenful of 99.
Lilo have relatively poor documentation.
Don't know its limit of maximum bootable entries yet but it boots all my 45+ system.
No revalidation on its /boot/grub/menu.lst and so can have unbootable entries which can be used to good effect. (by arranging empty partitions there, then install new distros inside and still have them bootable without altering the MBR.)
Can be used without an operating system and loaded to Grub prompt and then boot any system manually. Therefore Grub on its own in a devoted partition, a floppy or a CD is a formidable tool to solve booting problems.
Grub Manual is one of the best documentation I have come across. (just google "Grub Manual")
I have to put my vote on Grub to be easier to setup and trouble shoot. In fact I would go as far as saying with Grub you seldom need to trouble shoot.
A 5Gb space is all that is required for a distro to intsall in and Linux is free. Some ask considerable less.
The 45+ systems include all MS systems I could find in the house.
I couldn't possibly offer the above summary from my 14 months in Linux without installing a few of them and come to know their characteristics. When I started I knew nothing about booting and with just a XP.
What is the problem of having the opportunity to know all different type of desktops, media software, browsers, hardware detection programs and various functionalities for free? All of them can be booted by Grub, in the hard disk or with just a floppy.
I predict many Linux users will keep more distros in their machine once they realise to a Linux boot loader booting 100 systems is just as easy and simple as booting one.